Chapter 15 Study Guide
Match the components involved with ER transport with the appropriate cellular location. Locations can be used more than once, or not at all. 1. signal-recognition particle 2. protein translocator 3. mRNA 4. SRP receptor 5. active site of signal peptidase
1. cytosol 2. ER membrane 3. cytosol 4. ER membrane 5. ER lumen
Which of the following statements about membrane-enclosed organelles is true? 1. in a typical cell, the area of the ER membrane far exceeds the area of plasma membrane. 2. The nucleus is the only organelle that is surrounded by a double membrane. 3. Other than the nucleus, most organelles are small and thus, in a typical cell, only about 10% of a cell's volume is occupied by membrane-enclosed organelles; the other 90% of the cell volume is cytosol. 4. The nucleus is the only cell that contains DNA.
1. in a typical cell, the area of the ER membrane far exceeds the area of plasma membrane.
Name three possible fates for an endocytosed molecule that has reached the endosome.
1. recycled to the original membrane 2. destroyed in the lysosome 3. transcytosed across the cell to a different membrane
Which of the following statements about the ER is false? 1. The ER is the major site for new membrane synthesis in the cell. 2. proteins to be delivered to the ER lumen are synthesized on smooth ER. 3. Steroid hormones are synthesized in the smooth ER. 4. The ER membrane is contiguous with the outer nuclear membrane.
2. proteins to be delivered to the ER lumen are synthesized on smooth ER. proteins to be delivered to the ER lumen are synthesized in the rough ER.
the budding of clathrin-coated vesicles from eukaryotic plasma membrane fragments can be observed when adaptins, clathrin, and dynamin-gtp are added to the membrane preparation. What would you observe if you omitted (a) adaptins, (b) clathrin, or (c) dynamin? (d) What would you observe if the plasma membrane fragments were from a prokaryotic cell?
A. Clathrin coats cannot assemble in the absence of adaptins that link the clathrin to the membrane. At high clathrin concentrations and under the appropriate ionic conditions, clathrin cages assemble in solution, but they are empty shells, lacking other proteins, and they contain no membrane. This shows that the information to form clathrin baskets is contained in the clathrin molecules themselves, which are therefore able to self- assemble. B. Without clathrin, adaptins still bind to receptors in the membrane, but no clathrin coat can form and thus no clathrin-coated pits or vesicles are produced. C. Deeply invaginated clathrin-coated pits form on the membrane, but they do not pinch off to form closed vesicles. D. Prokaryotic cells do not perform endocytosis. A prokaryotic cell therefore does not contain any receptors with appropriate cytosolic tails that could mediate adaptin binding. Therefore, no clathrin can bind and no clathrin coats can assemble.
Which of the following statements about vesicle budding from the Golgi is false? A. Clathrin molecules are important for binding to and selecting cargoes for transport. B. Adaptins interact with clathrin. C. Once vesicle budding occurs, clathrin molecules are released from the vesicle. D. Clathrin molecules act at the cytosolic surface of the Golgi membrane.
A. Clathrin molecules are important for binding to and selecting cargoes for transport.
Plasma membrane proteins are inserted into the membrane in the ______A____________. The address information for protein sorting in a eukaryotic cell is contained in the _______B___________ of the proteins. Proteins enter the nucleus in their _________C_________ form. Proteins that remain in the cytosol do not contain a _______D___________. Proteins are transported into the Golgi apparatus via _________E_________. The proteins transported into the endoplasmic reticulum by _________F_________ are in their _________G_________ form.
A. ER B. Amino acids C. Folded D. Sorting signal E. Transport vesicles F. Protein translocator G. Unfolded
Using genetic engineering techniques, you have created a set of proteins that contain two (and only two) conflicting signal sequences that specify different compartments. Predict which signal would win out for the following combinations. Explain your answers. A. Signals for import into the nucleus and import into the ER. B. Signals for export from the nucleus and import into the mitochondria. C. Signals for import into mitochondria and retention in the ER.
A. ER B. Mitochondria C. Mitochondria
What is the role of the nuclear localization sequence in a nuclear protein? A. It is bound by cytoplasmic proteins that direct the nuclear protein to the nuclear pore. B. It is a hydrophobic sequence that enables the protein to enter the nuclear membranes. C. It aids in protein unfolding so that the protein can thread through nuclear pores. D. It prevents the protein from diffusing out of the nucleus through nuclear pores.
A. It is bound by cytoplasmic proteins that direct the nuclear protein to the nuclear pore.
Which of the following statements about peroxisomes is false? A. Most peroxisomal proteins are synthesized in the ER. B. Peroxisomes synthesize phospholipids for the myelin sheath. C. Peroxisomes produce hydrogen peroxide. D. Vesicles that bud from the ER can mature into peroxisomes.
A. Most peroxisomal proteins are synthesized in the ER.
Which of the following statements about phagocytic cells in animals is false? A. Phagocytic cells are important in the gut to take up large particles of food. B. Phagocytic cells scavenge dead and damaged cells and cell debris. C. Phagocytic cells can engulf invading microorganisms and deliver them to their lysosomes for destruction. D. Phagocytic cells extend pseudopods that surround the material to be ingested.
A. Phagocytic cells are important in the gut to take up large particles of food.
Eukaryotic cells are continually taking up materials from the extracellular space by the process of endocytosis. One type of endocytosis is _________A_________, which uses _________B_________ proteins to form small vesicles containing fluids and molecules. After these vesicles have pinched off from the plasma membrane, they will fuse with the _________C_________, where materials that are taken into the vesicle are sorted. A second type of endocytosis is _________D_________, which is used to take up large vesicles that can contain microorganisms and cellular debris. Macrophages are especially suited for this process, as they extend _________E_________ (sheetlike projections of their plasma membrane) to surround the invading microorganisms
A. Pinocytosis B. Clathrin C. Endosome D. Phagocytosis E. Pseudopods.
A. predict the membrane orientation of a protein that is synthesized with an uncleaved, internal signal sequence but does not contain a stop-transfer sequence. B. similarly, predict the membrane orientation of a protein that is synthesized with an n-terminal cleaved signal sequence followed by a stop-transfer sequence, followed by a start-transfer sequence. C. What arrangement of signal sequences would enable the insertion of a multipass protein with an odd number of transmembrane segments?
A. The internal signal sequence functions as a membrane anchor. Because there is no stop-transfer sequence, however, the C-terminal end of the protein continues to be translocated into the ER lumen. The resulting protein therefore has its N-terminal domain in the cytosol, followed by a single transmembrane segment, and a C-terminal domain in the ER lumen. B. The N-terminal signal sequence initiates translocation of the N-terminal domain of the protein until translocation is stopped by the stop-transfer sequence. A cytosolic domain is synthesized until the start-transfer sequence initiates translocation again. The situation now resembles that described in (A), and the C-terminal domain of the protein is translocated into the lumen of the ER. The resulting protein therefore spans the membrane twice. Both its N-terminal and C-terminal domains are in the ER lumen, and a loop domain between the two transmembrane regions is exposed in the cytosol. C. It would need a cleaved signal sequence, followed by an internal stop-transfer sequence, followed by pairs of start- and stop-transfer sequences.
Which of the following statements about secretion is true? A. The membrane of a secretory vesicle will fuse with the plasma membrane when it discharges its contents to the cell's exterior. B. Vesicles for regulated exocytosis will not bud off the trans Golgi network until the appropriate signal has been received by the cell. C. The signal sequences of proteins destined for constitutive exocytosis ensure their packaging into the correct vesicles. D. Proteins destined for constitutive exocytosis aggregate as a result of the acidic pH of the trans Golgi network.
A. The membrane of a secretory vesicle will fuse with the plasma membrane when it discharges its contents to the cell's exterior.
name the membrane-enclosed compartments in a eukaryotic cell where each of the functions listed below takes place. A. photosynthesis B. transcription C. oxidative phosphorylation D. modification of secreted proteins E. steroid hormone synthesis F. degradation of worn-out organelles G. new membrane synthesis H. breakdown of lipids and toxic molecules.
A. chloroplast B. nucleus C. mitochondrion D. smooth ER E. smooth ER F. lysosome G. Golgi apparatus and rough ER H. peroxisome
The ___A____makes up about half of the total cell volume of a typical eukaryotic cell. Ingested materials within the cell will pass through a series of compartments called ____B_______ in their way to the _____C_____, which contains digestive enzymes and will ultimately degrade the particles and macromolecules taken into the cell and will also degrade worn-out organelles. the ______D______ has a cis and trans face and receives proteins and lipids from the ______E______, a system of interconnected sacs and tubes of membranes that typically extends throughout the cell.
A. cytosol B. endosomes C. lysosomes D. Golgi apparatus E. endoplasmic reticulum
Proteins are transported out of a cell via the __________A________ or ___________B_______ pathway. Fluids and macromolecules are transported into the cell via the _________C_________ pathway. All proteins being transported out of the cell pass through the ______D____________ and the ________E__________. Transport vesicles link organelles of the __________F________ system. The formation of _______G___________ in the endoplasmic reticulum stabilizes protein structure.
A. secretory B. Exocytic C. Endocytic D. ER E. Golgi apparatus F. endomembrane G. disulfide bonds
A large protein that passes through the nuclear pore must have an appropriate _________. A. sorting sequence, which typically contains the positively charged amino acids lysine and arginine. B. sorting sequence, which typically contains the hydrophobic amino acids leucine and isoleucine. C. sequence to interact with the nuclear fibrils. D. Ran-interacting protein domain.
A. sorting sequence, which typically contains the positively charged amino acids lysine and arginine.
Proteins that are fully translated in the cytosol and lack a sorting signal will end up in ____. A. the cytosol. B. the mitochondria. C. the interior of the nucleus. D. the nuclear membrane.
A. the cytosol
consider the v-SNAREs that direct transport vesicles from the trans Golgi network to the plasma membrane. They, like all other v-SNAREs, are membrane proteins that are integrated into the membrane of the ER during their biosynthesis and are then carried by transport vesicles to their destination. Thus, transport vesicles budding from the ER contain at least two kinds of v-SNAREs - those that target the vesicles to the cis Golgi cisternae, and those that are in transit to the trans Golgi network to be packaged in different transport vesicles destined for the plasma membrane. A. Why might this be a problem? B. Suggest possible ways in which the cell might solve it.
A. the problem is that vesicles having two different kinds of v-SNAREs in their membrane could dock on either of two different membranes. B. the actual answer is not known, but we can predict that cells must have ways of turning the docking ability of SNAREs on and off.
What would happen in each of the following cases? Assume in each case that the protein involved is a soluble protein, not a membrane protein. A. You add a signal sequence (for the ER) to the N-terminal end of a normally cytosolic protein. B. You change the hydrophobic amino acids in an ER signal sequence into charged amino acids. C. You change the hydrophobic amino acids in an ER signal sequence into other hydrophobic amino acids. D. You move the N-terminal ER signal sequence to the C-terminal end of the protein.
A. transported into the ER lumen. B. altered signal sequence not recognized, remains in the cytosol. C. Into ER; distribution of hydrophobic aa important, not actual sequence. D. Not into ER;C-terminus is last made, so therefore not recognized by SRP complex.
What would you expect to happen in cells that secrete large amounts of protein through the regulated secretory pathway if the ionic conditions in the er lumen could be changed to resemble those in the lumen of the trans golgi network?
Aggregates of the secretory proteins would form in the ER, just as they do in the trans Golgi network. As the aggregation is specific for secretory proteins, ER proteins would be excluded from the aggregates. The aggregates would eventually be degraded.
The lipid bilayer of the inner and outer nuclear membranes forms a continuous sheet, joined around the nuclear pores. as membranes are two- dimensional fluids, this would imply that membrane proteins can diffuse freely between the two nuclear membranes. Yet each of these two nuclear membranes has a different protein composition, reflecting different functions. How could you reconcile this apparent contradiction?
Although the nuclear envelope forms one continuous membrane, it has specialized regions that contain special proteins and have a characteristic appearance. One such specialized region is the inner nuclear membrane. Membrane proteins can indeed diffuse between the inner and outer nuclear membranes, at the connections formed around the nuclear pores. Those proteins with particular functions in the inner membrane, however, are usually anchored there by their interaction with other components such as chromosomes and the nuclear lamina.
Explain how an mRNA molecule can remain attached to the ER membrane while individual ribosomes translating it are released and rejoin the cytosolic pool of ribosomes after each round of translation.
An mRNA molecule is attached to the ER membrane by the ribosomes translating it. This ribosome population, however, is not static; the mRNA is continuously moved through the ribosome. Those ribosomes that have finished translation dissociate from the 3ʹ end of the mRNA and from the ER membrane, but the mRNA itself remains bound by other ribosomes, newly recruited from the cytosolic pool, that have attached to the 5ʹ end of the mRNA and are still translating the mRNA. Depending on its length, there are about 10-20 ribosomes attached to each membrane-bound mRNA molecule
Which of the following statements about transport into mitochondria and chloroplasts is false? A. The signal sequence on proteins destined for these organelles is recognized by a receptor protein in the outer membrane of these organelles. B. After a protein moves through the protein translocator in the outer membrane of these organelles, the protein diffuses in the lumen until it encounters a protein translocator in the inner membrane. C. Proteins that are transported into these organelles are unfolded as they are being transported. D. Signal peptidase will remove the signal sequence once the protein has been imported into these organelles.
B. After a protein moves through the protein translocator in the outer membrane of these organelles, the protein diffuses in the lumen until it encounters a protein translocator in the inner membrane.
You are working in a biotech company that has discovered a small-molecule drug called H5434. H5434 binds to LDL receptors when they are bound to cholesterol. H5434 binding does not alter the conformation of the LDL receptor's intracellular domain. Interestingly, in vitro experiments demonstrate that addition of H5434 increases the affinity of LDL for cholesterol and prevents cholesterol from dissociating from the LDL receptor even in acidic conditions. Which of the following is a reasonable prediction of what may happen when you add H5434 to cells? A. Cytosolic cholesterol levels will remain unchanged relative to normal cells. B. Cytosolic cholesterol levels will decrease relative to normal cells. C. The LDL receptor will remain on the plasma membrane. D. The uncoating of vesicles will not occur.
B. Cytosolic cholesterol levels will decrease relative to normal cells.
Which of the following statements about the protein quality control system in the ER is false? A. Chaperone proteins help misfolded proteins fold properly. B. Proteins that are misfolded are degraded in the ER lumen. C. Protein complexes are checked for proper assembly before they can exit the ER. D. A chaperone protein will bind to a misfolded protein to retain it in the ER.
B. Proteins that are misfolded are degraded in the ER lumen.
Molecules to be packaged into vesicles for transport are selected by: A. clathrin. B. adaptins. C. dynamin. D. SNAREs.
B. adaptins
Which of the following statements is true? A. Lysosomes are believed to have originated from the engulfment of bacteria specialized for digestion. B. The nuclear membrane is thought to have arisen from the plasma membrane invaginating around the DNA. C. Because bacteria do not have mitochondria, they cannot produce ATP in a membrane-dependent fashion. D. Chloroplasts and mitochondria share their DNA.
B. the nuclear membrane is thought to have arisen from the plasma membrane invaginating around the DNA.
An individual transport vesicle : A. contains only one type of protein in its lumen. B. will fuse with only one type of membrane. C. is endocytic if it is traveling toward the plasma membrane. D. is enclosed by a membrane with the same lipid and protein composition as the membrane of the donor organelle.
B. will fuse with only one type of membrane.
Which of the following statements about the unfolded protein response (UPR) is false? A. Activation of the UPR results in the production of more ER membrane. B. Activation of the UPR results in the production of more chaperone proteins. C. Activation of the UPR occurs when receptors in the cytoplasm sense misfolded proteins. D. Activation of the UPR results in the cytoplasmic activation of gene regulatory proteins.
C. Activation of the UPR occurs when receptors in the cytoplasm sense misfolded proteins.
Which of the following statements is true? A. The signal sequences on mitochondrial proteins are usually at the C-terminus. B. Most mitochondrial proteins are not imported from the cytosol but are synthesized inside the mitochondria. C. Chaperone proteins in the mitochondria facilitate the movement of proteins across the outer and inner mitochondrial membranes. D. Mitochondrial proteins cross the membrane in their native, folded state.
C. Chaperone proteins in the mitochondria facilitate the movement of proteins across the outer and inner mitochondrial membranes.
Which of the following statements about nuclear transport is true? A. mRNAs and proteins transit the nucleus through different types of nuclear pores. B. Nuclear import receptors bind to proteins in the cytosol and bring the proteins to the nuclear pores, where the proteins are released from the receptors into the pores for transit into the nucleus. C. Nuclear pores have water-filled passages that small, water-soluble molecules can pass through in a nonselective fashion. D. Nuclear pores are made up of many copies of a single protein.
C. Nuclear pores have water-filled passages that small, water-soluble molecules can pass through in a nonselective fashion.
Your friend works in a biotechnology company and has discovered a drug that blocks the ability of Ran to exchange GDP for GTP. What is the most likely effect of this drug on nuclear transport? A. Nuclear transport receptors would be unable to bind cargo. B. Nuclear transport receptors would be unable to enter the nucleus. C. Nuclear transport receptors would be unable to release their cargo in the nucleus. D. Nuclear transport receptors would interact irreversibly with the nuclear pore fibrils.
C. Nuclear transport receptors would be unable to release their cargo in the nucleus.
Which of the following statements is true? A. Proteins destined for the ER are translated by a special pool of ribosomes whose subunits are always associated with the outer ER membrane. B. Proteins destined for the ER translocate their associated mRNAs into the ER lumen where they are translated. C. Proteins destined for the ER are translated by cytosolic ribosomes and are targeted to the ER when a signal sequence emerges during translation. D. Proteins destined for the ER are translated by a pool of cytosolic ribosomes that contain ER-targeting sequences that interact with ER-associated protein translocators.
C. Proteins destined for the ER are translated by cytosolic ribosomes and are targeted to the ER when a signal sequence emerges during translation.
Which of the following statements about vesicular membrane fusion is false? A. Membrane fusion does not always immediately follow vesicle docking. B. The hydrophilic surfaces of membranes have water molecules associated with them that must be displaced before vesicle fusion can occur. C. The GTP hydrolysis of the Rab proteins provides the energy for membrane fusion. D. The interactions of the v-SNAREs and the t-SNAREs pull the vesicle membrane and the target organelle membrane together so that their lipids can intermix.
C. The GTP hydrolysis of the Rab proteins provides the energy for membrane fusion.
Cells have oligosaccharides displayed on their cell surface that are important for cell-cell recognition. Your friend discovered a transmembrane glycoprotein, GP1, on a pathogenic yeast cell that is recognized by human immune cells. He decides to purify large amounts of GP1 by expressing it in bacteria. To his purified protein he then adds a branched 14-sugar oligosaccharide to the asparagine of the only Asn-X-Ser sequence found on GP1 (Figure Q15-48). Unfortunately, immune cells do not seem to recognize this synthesized glycoprotein. Which of the following statements is a likely explanation for this problem? A. The oligosaccharide should have been added to the serine instead of the asparagine. B. The oligosaccharide should have been added one sugar at a time. C. The oligosaccharide needs to be further modified before it is mature. D. The oligosaccharide needs a disulfide bond.
C. The oligosaccharide needs to be further modified before it is mature.
Most proteins destined to enter the endoplasmic reticulum _________. A. are transported across the membrane after their synthesis is complete. B. are synthesized on free ribosomes in the cytosol. C. begin to cross the membrane while still being synthesized. D. remain within the endoplasmic reticulum.
C. begin to cross the membrane while still being synthesized.
Vesicles from the ER enter the Golgi at the: A. medial cisternae. B. trans Golgi network. C. cis Golgi network. D. trans cisternae.
C. cis Golgi network.
Your friend has just joined a lab that studies vesicle budding from the Golgi and has been given a cell line that does not form mature vesicles. He wants to start designing some experiments but wasn't listening carefully when he was told about the molecular defect of this cell line. He's too embarrassed to ask and comes to you for help. He does recall that this cell line forms coated pits but vesicle budding and the removal of coat proteins don't happen. Which of the following proteins might be lacking in this cell line? A. clathrin B. Rab C. dynamin D. adaptin
C. dynamin
Signal sequences that direct proteins to the correct compartment are _________. A. added to proteins through post-translational modification. B. added to a protein by a protein translocator. C. encoded in the amino acid sequence and sufficient for targeting a protein to its correct destination. D. always removed once a protein is at the correct destination.
C. encoded in the amino acid sequence and sufficient for targeting a protein to its correct destination.
In which cellular location would you expect to find ribosomes translating mRNAs that encode ribosomal proteins? A. the nucleus B. on the rough ER C. in the cytosol D. in the lumen of the ER
C. in the cytosol
Which of the following organelles is not part of the endomembrane system? A. Golgi apparatus B. The nucleus C. mitochondria D. lysosomes
C. mitochondria
Proteins that are fully translated in the cytosol do not end up in _______. A. the cytosol. B. the mitochondria. C. the interior of the nucleus. D. transport vesicles.
C. the interior of the nucleus
New plasma membrane reaches the plasma membrane by the [regulated/constitutive] exocytosis pathway. New plasma membrane proteins reach the plasma membrane by the [regulated/constitutive] exocytosis pathway. Insulin is secreted from pancreatic cells by the [regulated/constitutive] exocytosis pathway. The interior of the trans Golgi network is [acidic/alkaline]. Proteins that are constitutively secreted [aggregate/do not aggregate] in the trans Golgi network.
Constitutive Constitutive Regulated Acidic Do not aggregate
If a lysosome breaks, what protects the rest of the cell from lysosomal enzymes?
Cytosolic pH.
Which of the following choices reflects the appropriate order of locations through which a protein destined for the plasma membrane travels? A. lysosome --> endosome --> plasma membrane B. ER --> lysosome --> plasma membrane C. Golgi --> lysosome --> plasma membrane D. ER --> Golgi --> plasma membrane
D. ER --> Golgi --> plasma membrane
Different glycoproteins can have a diverse array of oligosaccharides. Which of the statements below about this diversity is true? A. Extensive modification of oligosaccharides occurs in the extracellular space. B. Different oligosaccharides are covalently linked to proteins in the ER and the Golgi. C. A diversity of oligosaccharyl transferases recognizes specific protein sequences, resulting in the linkage of a variety of oligosaccharides to proteins. D. Oligosaccharide diversity comes from modifications that occur in the ER and the Golgi of the 14-sugar oligosaccharide added to the protein in the ER.
D. Oligosaccharide diversity comes from modifications that occur in the ER and the Golgi of the 14-sugar oligosaccharide added to the protein in the ER.
You are interested in Fuzzy, a soluble protein that functions within the ER lumen. Given that information, which of the following statements must be true? A. Fuzzy has a C-terminal signal sequence that binds to SRP. B. Only one ribosome can be bound to the mRNA encoding Fuzzy during translation. C. Fuzzy must contain a hydrophobic stop-transfer sequence. D. Once the signal sequence from Fuzzy has been cleaved, the signal peptide will be ejected into the ER membrane and degraded.
D. Once the signal sequence from Fuzzy has been cleaved, the signal peptide will be ejected into the ER membrane and degraded.
Which of the following statements about a protein in the lumen of the ER is false? A. A protein in the lumen of the ER is synthesized by ribosomes on the ER membrane. B. Some of the proteins in the lumen of the ER can end up in the extracellular space. C. Some of the proteins in the lumen of the ER can end up in the lumen of an organelle in the endomembrane system. D. Some of the proteins in the lumen of the ER can end up in the plasma membrane.
D. Some of the proteins in the lumen of the ER can end up in the plasma membrane.
Which of the following protein families are not involved in directing transport vesicles to the target membrane? A. SNAREs B. Rabs C. tethering proteins D. adaptins
D. adaptins
After isolating the rough endoplasmic reticulum from the rest of the cytoplasm, you purify the RNAs attached to it. Which of the following proteins do you expect the RNA from the rough endoplasmic reticulum to encode? A. soluble secreted proteins B. ER membrane proteins C. plasma membrane proteins D. all of the above
D. all of the above
Where are proteins in the chloroplast synthesized? A. in the cytosol B. in the chloroplast C. on the endoplasmic reticulum D. in both the cytosol and the chloroplast
D. in both the cytosol and the chloroplast
N-linked oligosaccharides on secreted glycoproteins are attached to: A. nitrogen atoms in the polypeptide backbone. B. the serine or threonine in the sequence Asn-X-Ser/Thr. C. the N-terminus of the protein. D. the asparagine in the sequence Asn-X-Ser/Thr.
D. the asparagine in the sequence Asn-X-Ser/Thr.
Why do eukaryotic cells require a nucleus as a separate compartment when prokaryotic cells can manage perfectly well without?
Eukaryotic gene expression is more complicated than prokaryotic gene expression. In particular, prokaryotic cells do not have introns that interrupt the coding sequences of their genes, so that an mRNA can be translated immediately after it is transcribed, without a need for further processing. In fact, in prokaryotic cells, ribosomes start translating most mRNAs before transcription is finished. This would have disastrous consequences in eukaryotic cells, because most RNA transcripts have to be spliced before they can be translated. The nuclear envelope separates the transcription and translation processes in space and time: a primary RNA transcript is held in the nucleus until it is properly processed to form an mRNA, and only then is it allowed to leave the nucleus so that ribosomes can translate it.
T/F N-linked sugar chains are found on glycoproteins that face the cell surface, as well as on glycoproteins that face the lumen of the ER, trans Golgi network, and mitochondria.
False. Mitochondria do not participate in vesicular transport, and therefore N-linked glycoproteins, which are exclusively assembled in the ER, cannot be transported to mitochondria.
T/F The amino acid sequence Leu-His-Leu-Asp-Ala-Gln-Ser-Lys-Leu-Ser-Ser is a signal sequence that directs proteins to the ER.
False. The signal sequences that direct proteins to the ER contain a core of eight or more hydrophobic amino acids. The sequence shown here contains many hydrophilic amino acid side changes, including the charged amino acids His, Arg, Asp, Lys and the uncharged hydrophilic amino acids Gln and Ser.
T/F Lysosomes digest only substances that have been taken up by cells by endocytosis.
False. lysosomes also digest internal organelles by autophagy.
Why might it be advantageous to add a preassembled block of 14 sugar residues to a protein in the er, rather than building the sugar chains step-by-step on the surface of the protein by the sequential addition of sugars by individual enzymes?
The preassembled sugar chain allows better quality control. The assembled oligosaccharide chains can be checked for accuracy before they are added to the protein; if a mistake were made in adding sugars individually to the protein, the whole protein would have to be discarded. Because far more energy is used in building a protein than in building a short oligosaccharide chain, this is a much more economical strategy. This difficulty becomes apparent as the protein moves to the cell surface: although sugar chains are continually modified by enzymes in various compartments of the secretory pathway, these modifications are often incomplete and result in considerable heterogeneity of the glycoproteins that leave the cell. This heterogeneity is largely due to the restricted access that the enzymes have to the sugar trees attached to the surface of proteins. The heterogeneity also explains why glycoproteins are more difficult to study and purify than nonglycosylated proteins.
iron (Fe) is an essential trace metal that is needed by all cells. it is required, for example, for synthesis of the heme groups and iron-sulfur centers that are part of the active site of many proteins involved in electron-transfer reactions; it is also required in hemoglobin, the main protein in red blood cells. iron is taken up by cells by receptor-mediated endocytosis. the iron-uptake system has two components: a soluble protein called transferrin, which circulates in the bloodstream; and a transferrin receptor—a transmembrane protein that, like the LDL receptor, is continually endocytosed and recycled to the plasma membrane. Fe ions bind to transferrin at neutral pH but not at acidic pH. transferrin binds to the transferrin receptor at neutral pH only when it has an Fe ion bound, but it binds to the receptor at acidic pH even in the absence of bound iron. From these properties, describe how iron is taken up, and discuss the advantages of this elaborate scheme.
Transferrin without Fe bound does not interact with its receptor and circulates in the bloodstream until it catches an Fe ion. Once iron is bound, the iron- transferrin complex can bind to the transferrin receptor on the surface of a cell and be endocytosed. Under the acidic conditions of the endosome, the transferrin releases its iron, but the transferrin remains bound to the transferrin receptor, which is recycled back to the cell surface, where it encounters the neutral pH environment of the blood. The neutral pH causes the receptor to release the transferrin into the circulation, where it can pick up another Fe ion to repeat the cycle. The iron released in the endosome moves on to lysosomes, from where it is transported into the cytosol. The system allows cells to take up iron efficiently even though the concentration of iron in the blood is extremely low. The iron bound to transferrin is concentrated at the cell surface by binding to transferrin receptors; it becomes further concentrated in clathrin-coated pits, which collect the transferrin receptors. In this way, transferrin cycles between the blood and endosomes, delivering the iron that cells need to grow.
T/F Ribosomes are cytoplasmic structures that, during protein synthesis, become linked by an mRNA molecule to form polyribosomes.
True
T/F Transport vesicles deliver proteins and lipids to the cell surface.
True
T/F If the delivery of prospective lysosomal proteins from the trans Golgi network to the late endosomes were blocked, lysosomal proteins would be secreted by the constitutive secretion pathways.
True. Lysosomal proteins are selected in the trans Golgi network and packaged into transport vesicles that deliver them to the late endosome. If not selected, they would enter by default into transport vesicles that move constitutively to the cell surface.
T/F All transport vesicles in the cell must have a v-SNARE protein in their membrane.
True. Otherwise they could not dock at the correct target membrane or recruit a fusion complex to a docking site.
phagocytic cell
a cell such as a macrophage or neutrophil that is specialized to take up particles and microorganisms by phagocytosis
rab protein
a family of small GTP-binding proteins present on the surfaces of transport vesicles and organelles that serves as a molecular marker to help ensure that transport vesicles fuse only with the correct membrane
What would happen to proteins bound for the nucleus if there were insufficient energy to transport them?
a single, incomplete round of nuclear import would occur. because nuclear transport is fueled by GTP hydrolysis, under conditions of insufficient energy, GTP would be used up and no Ran-GTP would be available to unload the cargo protein from its nuclear import receptor upon arrival in the nucleus. unable to release its cargo, the nuclear import receptor would be stuck at the nuclear pore and not return to the cytosol. because the nuclear cargo protein is not released; it would not be functional, and no further import could occur.
signal sequence
amino acid sequence that directs a protein to a specific location in the cell, such as the nucleus or mitochondria
membrane-enclosed organelle
any organelle in a eukaryotic cell that is surrounded by a lipid bilayer, for example, ER, Golgi and lysosome
nuclear pore
channel through which selected large molecules move between the nucleus and cytoplasm
nuclear envelope
double membrane surrounding the nucleus. Consists of outer and inner membranes, perforated by nuclear pores
You discover a fungus that contains a strange star-shaped organelle not found in any other eukaryotic cell you have seen. On further investigation, you find the following: 1. The organelle possesses a small genome in its interior. 2. The organelle is surrounded by two membranes. 3. Vesicles do not pinch off from the organelle membrane. 4. The interior of the organelle contains proteins similar to those of many bacteria. 5. The interior of the organelle contains ribosomes.
from an engulfed bacterium - double membrane, genome, ribosomes, proteins similar to that of bacteria.
Endomembrane system
interconnected network of membrane-enclosed organelles in a eukaryotic cell; includes the ER, Golgi, lysosomes, peroxisomes, and endosomes
Endoplasmic reticulum (ER)
labyrinthine membrane-enclosed compartment in the cytoplasm of eukaryotic cells where lipids and proteins are made
Autophagy
mechanism by which a cell "eats itself", digesting molecules and organelles that are damaged or obsolete
receptor-mediated endocytosis
mechanism of selective uptake of material by animal cells in which a macromolecule binds to a receptor in the plasma membrane and enters the cell in a clathrin-coated vesicle
transport vesicle
membrane vesicle that carries proteins from one intracellular compartment to another, for example, from the endoplasmic reticulum to the Golgi apparatus
Endosome
membrane-enclosed compartment of a eukaryotic cell through which material ingested by endocytosis passes on its way to lysosomes
Golgi apparatus
membrane-enclosed organelle in eukaryotic cells that modifies the proteins and lipids made in the endoplasmic reticulum and sorts them for transport to other sites
secretory vesicle
membrane-enclosed organelle in which molecules destined for secretion are stored prior to release. sometimes called a secretory granule, because darkly staining contents make the organelle visible as a small solid object
Lysosomes
membrane-enclosed organelle that breaks down worn out proteins and organelles and other waste materials, as well as molecules taken up by endocytosis; contains digestive enzymes that are typically most active at the acid pH found inside these organelles
unfolded protein response (UPR)
molecular program triggered by accumulation of misfolded proteins in the endoplasmic reticulum. allows cells to expand the endoplasmic reticulum and produce more of the molecular machinery needed to restore proper protein folding and processing.
Chaperone protein
molecule that steers proteins along productive folding pathways, helping them to fold correctly and preventing them from forming aggregates inside the cell
vesicular transport
movement of material between organelles in the eukaryotic cell via membrane-enclosed vesicles
SNARE
one of a family of membrane proteins responsible for the selective fusion of vesicles with a target membrane inside the cell
Endocytosis
process by which cells take in materials through an invagination of the plasma membrane, which surrounds the ingested material in a membrane-enclosed vesicle
Exocytosis
process by which most molecules are secreted from a eukaryotic cell. These molecules are packaged in membrane-enclosed vesicles that fuse with the plasma membrane, releasing their contents to the outside
secretion
production and release of a substance from a cell
Clathrin
protein that makes up the coat of a type of transport vesicle that buds from either the Golgi apparatus (on the outward secretory pathway) or from the plasma membrane (on the inward endocytic pathway)
Compare and contrast protein import into the ER and into the nucleus. List at least two major differences in the mechanisms.
proteins are imported into the nucleus after they have been synthesized, folded, and if appropriate, assembled into complexes. in contrast, unfolded polypeptide chains are translocated into the ER as they are being made by the ribosomes.
rough endoplasmic reticulum
region of the endoplasmic reticulum associated with ribosomes and involved in the synthesis of secreted and membrane-bound proteins
peroxisome
small membrane-enclosed organelle that contains enzymes that degrade lipids and destroy toxins
Coated vesicle
small membrane-enclosed sac that wears a distinctive layer of proteins on its cytosolic surface. it is formed by pinching-off a protein-coated region of cell membrane
A particular type of Drosophila mutant becomes paralyzed when the temperature is raised. the mutation affects the structure of dynamin, causing it to be inactivated at the higher temperature. Indeed, the function of dynamin was discovered by analyzing the defect in these mutant fruit flies. The complete paralysis at the elevated temperature suggests that synaptic transmission between nerve and muscle cells is blocked. Suggest why signal transmission at a synapse might require dynamin. On the basis of your hypothesis, what would you expect to see in electron micrographs of synapses of flies that were exposed to the elevated temperature?
synaptic transmission involves the release of neurotransmitters by exocytosis. during this event, the membrane of the synaptic vesicle fuses with the plasma membrane of the nerve terminals. To make new synaptic vesicles, membranes must be retrieved from the plasma membrane by endocytosis. the endocytosis step is blocked if dynamin is defective, as the protein is required to pinch off the clatrhin-coated endocytic vesicles. the first clue to deciphering the role of dynamin came from electron micrographs of synapses of the mutant flies.
phagocytosis
the process by which particulate material is engulfed by a cell. prominent in predatory cells, such as Amoeba proteus, and in cells of the vertebrate immune system such as macrophages
Consider a protein that contains an ER signal sequence at its N-terminus and a nuclear localization sequence in its middle. What do you think the fate of this protein would be?
the protein is translocated into the ER. its ER signal sequence is recognized as soon as it emerges from the ribosome. the ribosome then becomes bound to the ER membrane, and the growing polypeptide chain is transferred through the ER translocation channel. The nuclear localization sequence is therefore never exposed to the cytosol. it will never encounter nuclear import receptors, and the protein will not enter the nucleus.
If you remove the ER retention signal from a protein that normally resides in the ER lumen, where do you predict the protein will ultimately end up? Explain your reasoning.
the protein would end up in the extracellular space, because it has no signals to be recognized by.
A protein that inhibits certain proteolytic enzymes (proteases) is normally secreted in to the bloodstream by liver cells. This inhibitor protein, antitrypsin, is absent from the bloodstream of patients who carry a mutation that results in a single amino acid change in the protein. Antitrypsin deficiency causes a variety of severe problems, particularly in lung tissue, because of the uncontrolled activity of proteases. Surprisingly, when the mutant antitrypsin is synthesized in the laboratory, it is as active as the normal antitrypsin at inhibiting proteases. Why, then, does the mutation cause the disease?
the single amino acid change causes the protein to misfold slightly, so that, although it is still active as protease inhibitor, it is prevented by chaperone proteins in the ER from exiting this organelle. it therefore accumulates in the ER lumen and is eventually degraded.
Some proteins shuttle back and forth between the nucleus and the cytosol. They need a nuclear export signal to get out of the nucleus. How do you suppose they get into the nucleus?
they must contain a nuclear localization signal as well. Proteins with nuclear export signals shuttle between the nucleus and the cytosol. An example is the A1 protein, which binds to mRNAs in the nucleus and guides them through the nuclear pores. Once in the cytosol, a nuclear localization signal ensures that the A1 protein is re-imported so that it can participate in the export of further mRNAs.
If membrane proteins are integrated into the ER membrane by means of the ER protein translocator (which itself is composed of membrane proteins), how do the first protein translocation channels become incorporated into the ER membrane?
this situation never arises in present day cells, although it must have posed a considerable problem for the first cells that evolved. new cell membranes are made by expansion of existing membranes, and the ER is never made de novo. there will always be an existing piece of ER with translocation channels to integrate new translocation channels. inheritance is therefore not limited to the propagation of the genome; a cell's organelles must also be passed from generation to generation. in fact, the ER translocation channels can be traced back to structurally relation translocation channels in the prokaryotic plasma membrane.
In a cell capable of regulated secretion, what are the three main classes of proteins that must be separated before they leave the trans Golgi network?
those destined for lysosomes (via late endosomes) those destined for secretory vesicles and those destined for immediate delivery to the cell surface (whether basolateral or apical)
pinocytosis
type of endocytosis in which soluble materials are taken up from the environment and incorporated into vesicles for digestion (literally "cell drinking"
